Methods of heat treating articles



B. GREENE Feb. 2, 1965 METHODS OF HEAT TREATING ARTICLES 2 Sheets-Sheet 1 Filed Dec. 28, 1960 Invzurag 45M G nee NE MW, UM

Feb. 2, 1965 B. GREENE 3,168,607

METHODS OF HEAT TREATING ARTICLES Filed Dec. 28, 1960 2 Sheets-Sheet 2 I VEN'roK $5M Gees/v5 United States Patent 3,168,607 METHODS OF HEAT TREATING ARTICLES Ben Greene, Braefoot Farm, Fossoway, Kinross-Shire, Scotland Filed Dec. 28, 1960, Ser. No. 78,958 5 Claims. (Cl. 266-5) This invention relates to methods of heat treating articles and particularly for heating metal articles and has an an object give rapid and even heat penetration for such purposes as normalising, stress relieving, annealing and brazing.

According to this invention a method of heat treating metal articles comprises enclosing them in an internally heated furnace chamber containing an inert atmosphere at a pressure greater than atmospheric pressure.

The inert gas may comprise hydrogen or argon, nitrogen, helium.

The pressure employed is preferably above 20 pounds per square inch absolute.

The articles may be subsequently cooled in an inert gas also at a pressure greater than atmospheric pressure.

The pressure of the gas during cooling may be above 30 pounds per square inch absolute.

A gas of the same composition may be employed both during the heating and cooling treatments. One method according to the invention comprises introducing the articles to be heat treated through an opening into a furnace chamber closing said opening so as to render the chamber pressure tight and vacuum tight, evacuating air from said chamber passing an inert gas into it, increasing the pressure of the inert gas in the chamber above atmospheric pressure, subjecting the articles to a heat treatment within the chamber at the required temperature, terminating the heat treatment, withdrawing the hot atmosphere from said chamber and introducing a cooling atmosphere and finally withdrawing the articles through said opening.

The withdrawn hot atmosphere may be passed through a cooler and reintroduced into said chamber for cooling purposes at a pressure above atmospheric pressure.

The same inert atmosphere may be circulated through said chamber during both the heating and cooling treatments.

After a first part of the heat treatment the inert atmosphere may be withdrawn and the heat treatment continued under vacuum.

During the heat treatment the pressurised inert gas may be circulated through the furnace chamber, first over heating means and then over the articles to be heated which are sheltered from direct radiation from the heating means.

The apparatus may comprise a furnace chamber which is pressure tight and vacuum tight. There are ar ranged in a closed circuit with the furnace chamber a refrigerator and a recirculating pump or fan, and a means to introduce a source of inert gas such as hydrogen under pressure.

The furnace chamber may comprise a metal shell which is lined internally with a relatively thin lining of insulating refractory material which may be in the form of bricks, which material is such that there is a temperature difference of over 1000 C. at atmospheric pressure between the hot and cold faces of the refractory when the interior of the furnace is at a temperature of the order of 1300 C. A suitable material for this purpose is a refractory supplied under the trademark of Morgan MI. 28. The metal shell may be surrounded at least opposite the hot zone by a jacket through which water is circulated to maintain the metal shell at a relatively cool temperature during the heating of the furnace.

The furnace can be heated by either or both of two 3,168,607 Patented F eb. 2, 1965 independent sources. For example, by heating elements in the furnace proper or by the heating elements in the passage through which the pressurised atmosphere is circulated in the heating cycle. To achieve the greatest uniformity of temperature it is essential to circulate the pressurised gas and by heating this gas in the passageway the unbalanced heating due to radiation is totally eliminated. Where, however, maximum thermal density is required both heating zones can be switched on as the pressurized atmosphere is circulated.

For cooling purposes the same pressurised atmosphere can be used in which case when the heating elements are switched off the circulating fan is kept running, and by means of a damper the heating duct is shut and the cool ing duct containing the refrigerator is opened.

The furnace is operated as follows.

The furnace chamber is evacuated by a vacuum pump so as to purge the furnace of air, and if necessary a purging gas such as hydrogen may be passed through the chamber for a time, the furnace chamber being eventually evacuated again. Subsequently hydrogen under pressure is pumped into the furnace chamber until a pressure of over 20 pounds per square inch absolute is built up and the heating of the furnace by electric elements is commenced and circulation of the hydrogen under pressure is effected by the circulating fan.

If as a result of pressure heating, the inert gas is occluded in the article or metal undergoing heat treatment, this occluded gas can be removed by creating vacuum conditions in the furnace at the lower cooling temperatures.

Initial heating and cooling of the articles in an atmosphere of pressurised gas results in the articles being heated or cooled by convection instead of by radiation as is the case in an evacuated furnace and this in turn ensures that the articles are heated and cooled more quickly and more evenly and also enables heating elements to be used which are themselves at a lower temperature than would be the case if the furnace were evacuated or not pressurised.

The following is a description of one form of furnace for carrying out the method described above, reference being made to the accompanying drawings in which:

FIGURE 1 is a vertical longitudinal section through the furnace, the right hand being a section on the line AA of FIGURE 2, the centre part on the line C-A of FIGURE 2 and the left hand end on the line BA of FIGURE 2;

FIGURE 2 is a section on the line 2-2 of FIG- URE l; 1

FIGURE 3 is a section on the line 3-3 of FIGURE 1 and FIGURE 4 shows diagrammatically the external equipment of an apparatus suitable for performing the method.

The furnace comprises a metal shell 10 closed at one end 11 and provided with a movable door 12 at the other end fitted with clamps (not shown) The metal shell is lined with refractory lining 13 which extends over the end wall 11. Likewise the door is provided with a refractory lining 14.

The furnace is cylindrical in form and is encircled by a cooling jacket 15. A supply pipe 16 for inert gas eX- tends through the top of the shell and lining into the furnace chamber 17. The pipe is connected outside the furnace in circuit with a high compression hydrogen storage vessel 42, a drier 44, a purifier 45, a telescopic gas holder 40 one or more cylinders of hydrogen under pressure 46 and a compressor 43 which circuit is controlled by suitable valve gears 47, 48, 49, 50 and 38.

As best seen in FIGURES 2 and 3 two passages 18 and 19 are provided beneath the hearth 20 both of which passages, at the end nearer the door 12, communicate with the main part of the furnace chamber 17. The passage 18 contains electric heating elements 21 whereas the passage 19 contains a heat exchanger 22 comprising a number of pipes through which cooling liquid may be circulated.

Both passages 18 and 19 at the opposite end to the door 12 communicate with a space 23 formed in the refractory lining at the closed end 11 of the shell. Either one or both passages may be brought into and out of communication with said space 23 by moving a pivoted plate valve 24 into and out of contacts with the ends of the passages. The plate valve is fixed to a spindle 25 which passes out of the furnace through a suitable gland 26 and may be oscillated by an operating lever 27.

The roof and side walls of the furnace chamber proper are also provided with electric heating elements 28.

Th space 23 communicates with the furnace chamber through a flared opening 29 opposite which is an impeller 30 fixed to a driving shaft 31. The shaft extends out through the end wall 11 of the furnace and the refractory lining and is driven by an external motor 32 through a suitable transmission 33.

A vacuum pipe 34 extends through the bottom of the furnace chamber into the cooling passage 19 and is connected to a suitable vacuum pump 37 which by means of a control valve 49 may deliver the extracted hydrogen to the telescopic gas holder 46 or to burn off at 51. A pressure gauge 36 communicates with the interior of the furnace as do also thermocouples (not shown).

It will be appreciated with the above arrangement that heating in the furnace can be carried out in an inert atmosphere either at, or above, or below atmospheric pressure or in vacuum by appropriate adjustment of the valve gear. The gas at the required pressure may be circulated through the furnace chamber by the impeller 30 either in a hot condition or a cool condition according to the ad justrnent of the plate valve 24.

Where a flow of hydrogen or inert gas is required to pass through the furnace during the heating cycle, as in brazing, this can be effected by opening a valve 38 associated with a burn off tube communicating with the aforesaid cooling passage 19. In the case Where a certain degree of pressure is required to be established under the last mentioned conditions this can be effected at the required pressure by means of a regulated pressure release valve 39 in the burn off tube 35. Gas may thus be released at 52, a control valve 38 allowing the gas to go either to pass out 52 or to pass through a suitable pipe 41 to the gas holder for re-use.

The aforesaid fan 30 and plate valve 34 can be made of ceramic, steel or molybdenum according to the condi tions for which the furnace has been designed.

The telescopic gas holder 40 is initially filled from the cylinders 46 and subsequent losses are made good from the same source. The gas from the holder 40 passes through a compressor 43, a purifier and a drier 44 in series to the high compression storage vessel 4-2.

Alternatively, particularly in the case of small furnaces the gas holder, the compressor and storage vessel may be omitted, and the pressurised gas from the cylinders could pass straight to the purifier and drier under pressure to the furnace.

Under certain conditions the impeller 30 can be used to create pressure as well as circulating the inert atmosphere or cooling atmosphere.

Alternatively two separate fans may be provided one to circulate the hot gas and another to circulate the cooling gas.

In such a case the swinging plate valve 24 may be dispensed With.

Also the heating passage 18 might be arranged over the ceiling of the furnace chamber in the hot zone and the cooling passage 19 under the hearth or vice versa.

It will further be apprecaitecl that the furnace could be designed so as to be vertically disposed instead of horizontally. Also the passages 18 and 19 and their associated heating elements and heat exchanger could be placed outside the furnace.

Advantages of the invention are that since the shell of the furnace is liquid cooled and since there is a high temperature difference occurring across a relatively thin layer of the insulating refractory material described, it is possible to purge the furnace chamber and to achieve a vacuum because there is a relatively small volume of the refractory to retain the gases in its pores. In addition, when the insulating refractory material is hot, gases locked in the material by capillary action are readily released, so that when the furnace chamber is evacuated the out gassing from the material is minimized. Though designed for pressure heating, the same furnace can operate as a vacuum furnace if desired in the same heat treatment cycle. Another advantage is that, due to the thin layer of refractory material, a comparatively small amount of heat is retained in the refractory material itself and thus the chamber can be heated and cooled more rapidly than if the refractory layer were thicker.

I claim:

1. A furnace for heat treating metal articles comprising in combination, a furnace chamber which is pressure and vacuum tight, a refractory bounded closed path within the furnace chamber including a refrigerator, a refractory bounded closed path within the furnace chamber including heating means for heating inert gas to a temperature of at least 1000 C., means to select either closed path, a pump for circulating gases through the selected path, means to selectively introduce a source of inert gas under pressure of over 20 pounds per square inch absolute, and means for selectively producing a vacuum in the chamber.

2. A furnace as defined in claim 1, wherein the chamber has a metal shell with a thin lining of refractory material producing a temperature difference of over 1000 C. at atmospheric pressure between the hot and cold faces of the refractory when the interior of the chamber is at a temperature in the order of 1300 C.

3. A furnace as defined in claim 2, wherein themetal shell is surrounded by a jacket and means is provided for circulating water through the jacket. I

4. A furnace as defined in claim 1 having further heating means in the furnace chamber.

5. A furnace as defined in claim 1 wherein the inert gas is selected from the group comprising hydrogen, helium, nitrogen and argon; and a compartment is located within a common portion of said selected paths for receiving said metal articles.

References Cited by the Examiner UNITED STATES PATENTS 10/27 Guibert l4816.7

OTHER REFERENCES Making, Shaping and Treating of Steel, by U.S.S., 6th edition, 1951.

DAVID L. RECK, Primary Examiner.

ROGER L. CAMPBELL, MARCUS U. LYONS,

Examiners. 

1. A FURNACE FOR HEAT TREATING METAL ARTICLES COMPRISING IN COMBINATION, A FURANCE CHAMBER WHICH IS PRESSURE AND VACUUM TIGHT, A REFRACTORY BOUNDED CLOSED PATH WITHIN THE FURNACE CHAMBER INCLUDING A REFRIGERATOR, A REFRACTORY BOUNDED CLOSED PATH WITHIN THE FURNACE CHAMBER INCLUDING HEATING MEANS FOR HEATING INERT GAS TO A TEMPERATURE OF AT LEAST 1000*C., MEANS TO SELECT EITHER CLOSED PATH, A PUMP FOR CIRCULATING GASES THROUGH THE SELECTED PATH, MEANS TO SELECTIVELY INTRODUCE A SOURCE OF INERT GAS UNDER PRESSURE OF OVER 20 POUNDS PER SQUARE INCH ABSOLUTE, AND MEANS FOR SELECTIVELY PRODUCING A VACUUM IN THE CHAMBER. 